As is well known, whenever two rotating machine shafts are coupled together, such as the shaft of an electric motor and the shaft of a pump, it is important that the shafts be aligned within predetermined tolerances. Such shafts, when in perfect alignment, have their extended center lines (axes of rotation) coinciding along a straight line. Misalignment can lead to vibration, excessive wear, and ultimate destruction of couplings, bearings, seals, gears and other components.
A number of shaft alignment methods are known, which generally have in common the use of suitable alignment fixtures, also termed alignment brackets. The alignment brackets are employed to measure particular relative displacements (also termed offsets) as the shafts typically are rotated together through one revolution, taking readings at various angular positions. Traditionally the shafts are stopped at the 0.degree., 90.degree., 180.degree. and 270.degree. angular positions to take readings. However, readings may be taken at a number of angular positions other than 0.degree., 90.degree., 180.degree. and 270.degree.. Each relative displacement is measured between a point referenced to one of the shafts by means of the alignment bracket and a point on the other shaft. Dial indicators are often employed, these dial indicators having a plunger which moves a hand on the face of the dial indicator.
The readings are then used to calculate machine moves which will bring the shafts into alignment. The 0.degree., 90.degree., 180.degree. and 270.degree. angular positions at which readings are conventionally taken lie in geometric planes in which either of the machines, for example the motor, may be moved for purposes of alignment. In particular, the mounting bolts of the machine may be loosened, and the machine may be either moved in a horizontal plane, or moved in a vertical plane by placing or removing shims under one or more of the feet of the machine, or both. There are well developed calculation methods and procedures known in the art for determining what machine moves are needed to achieve an acceptably aligned condition based on measurement of relative displacement data at the 0.degree., 90.degree., 180.degree. and 270.degree. positions mentioned.
In one type of alignment bracket, a base is firmly clamped, as by a chain, or otherwise affixed to one shaft, and an extension bar or arm extends laterally from the base in a direction generally parallel to the shafts across the coupling to a reference point. A device for measuring displacement of the reference point relative to the shaft, such as an analog dial indicator, is positioned so as to measure relative displacement in a radial direction from the reference point to the periphery of the other shaft when the shafts are positioned at the 0.degree., 90.degree., 80.degree. and 270.degree. angular positions. The position of the alignment bracket is then reversed so as to be fixedly referenced to the other shaft, establishing a reference point adjacent a point on the periphery of the one shaft, and the procedure is repeated. Alternatively, a pair of alignment brackets may be employed for simultaneous readings.
Because such alignment brackets are designed for use with a wide range of shaft sizes, they are typically provided with a chain long enough to be used with the largest shaft size. Thus, when used with the small shaft sizes, there is usually a significant length of free chain that hangs freely from the alignment bracket. This free chain can bump against the alignment bracket and the shaft when the shaft is rotated during the taking of measurements. Because of the sensitivity of the measuring device, particularly analog dial indicator devices, this bumping can cause errors in readings. In addition, the free length of chain is cumbersome and otherwise complicates the taking of measurements.
In addition to being extremely sensitive to contact, analog dial indicator type devices are difficult to zero, do not indicate the relative direction of displacement and have significant parallax errors.
Still another disadvantage of prior art alignment devices results from difficulty in aligning the devices so that accurate results are obtained. This is especially true when the co-rotating shafts are significantly different in size. Particularly, in such situations it difficult and time consuming to position alignment devices so that they both adequately clear the coupling and are substantially the same height above the coupling. Moreover, even if this is accomplished, mechanical limitations in the systems make it difficult and time consuming to ensure that the displacement of the plungers of the sensors is a true reflection of the condition of the shafts.
Yet another problem results from difficulties positioning the devices on the shafts so that they are spaced apart to correspond to the dimensions of the measuring device. This difficulty can result, for example, from clearance problems present in the machine environment.
Accordingly, there exists a need in the art for an improved mechanical shaft alignment system which is easier to set up and use and which reduces the error in measurement and other problems which inherently result from the design of prior art devices.